Revision: Water Chemistry ICSE ICSE Class 8 CISCE

Eutrophication: “Organic matter in sewage poured into water bodies generally results in excessive growth of algae – which deoxygenates water and produces deadening atmosphere there.”

Water with a large amount of dissolved solids is not potable or suitable for drinking. Such water is called saline water.

The temperature at which a liquid turns into solid when cooled is known as the freezing point. The freezing point of water is 0°C.

The temperature at which water boils and changes to steam is called as boiling point. The boiling point of water is 100°C at atmospheric pressure.

A homogeneous mixture of two or more substances, whose relative amounts may be changed within certain limits, is called a solution.

Suspension: A suspension is a heterogeneous mixture in which the solute particles do not dissolve but remain suspended throughout the bulk of the medium.

• Colloids: A colloid is a type of mixture where tiny particles of one substance are dispersed evenly throughout another substance. These particles are small but still larger than molecules or simple ions.
• Dispersed Phase: The phase that is dispersed or present in the colloidal particle shape is called the dispersed phase. 
• Dispersion medium: Dispersion medium is the continuous phase of a colloid. It is the phase in which the dispersed phase is distributed.

Crystallisation is a process by which crystals of a substance are obtained by cooling a hot saturated solution.

Water of crystallization refers to “the fixed number of water molecules which enter into a loose chemical combination with the substance when the substance is crystallized from its hot saturated solution, is called water of crystallization.” 

Water obtained from various sources does not easily form a lather with soap, rather it forms a white sticky scum or a precipitate. This water is called hard water.

The water present in different natural sources has different substances dissolved in it. The water drawn from certain sources forms a lather with soap rather easily. Such water is called soft water.

Oil spills are the accidental discharges of petroleum into oceans or estuaries. The sources of spills are overturned oil tankers, offshore oil mining, and oil refineries.

Water pollution means any change in the water quality which makes it unsuitable for use by humans and by other living organisms.

The amount of heat that is needed to raise the temperature of a unit mass of a substance by 1°C is called specific heat capacity.

Water is obtained naturally from ice sheets, ice caps, glaciers, icebergs, ponds, lakes, rivers, streams, and underground is called freshwater.

The collection of rainwater for direct use or for recharging the groundwater level for indirect use is called water harvesting.

• Water resources include surface water (rivers, lakes, ponds) and groundwater (water stored below the earth).
• 97% of Earth’s water is saline (oceans); only 0.03% is available as fresh surface water for direct use.
• Surface water mainly comes from rainfall and melting of glaciers, but is easily polluted.
• Groundwater forms by percolation of rainwater and is stored in aquifers; it is the largest usable freshwater source.
• India depends on monsoon rains, and proper water management is essential for agriculture and economic growth.

• The Water (Prevention and Control of Pollution) Act, 1974, was passed to safeguard water resources. Main sources — domestic sewage, industrial effluents and agricultural run-off.
• BOD (Biochemical Oxygen Demand) is the dissolved oxygen required by microorganisms to decompose organic matter in water. High BOD = high pollution = low oxygen = death of aquatic organisms.
• Algal bloom — excessive growth of planktonic algae due to excess nitrogen and phosphorus. Releases toxins and causes fish mortality. Water hyacinth (Eichhornia crassipes) — "Terror of Bengal" — is an invasive plant that chokes water bodies.
• Eutrophication — natural ageing of a lake by nutrient enrichment. When accelerated by human activities, it is called Cultural/Accelerated Eutrophication, depleting oxygen and killing aquatic life.
• Biomagnification — increase in concentration of toxic pollutants (DDT, mercury) at successive trophic levels. Non-degradable, accumulate in tissues and pass to the next trophic level.
• Thermal pollution — caused by thermal and nuclear power plants releasing hot coolant water, raising water temperature and killing aquatic flora and fauna.

• Proper maintenance of water bodies and avoiding the disposal of waste into them helps reduce sewage pollution.
• Use of pesticides and chemical fertilisers should be minimised to prevent water contamination.
• Sewage should be properly treated before being released into rivers and other water bodies.
• Ecological sanitation (Ecosan) uses dry composting toilets to recycle human waste into organic manure, reducing the need for fertilisers.
• Ecosan toilets save water, prevent groundwater contamination, and are useful in water-scarce areas.
• Recycling of sewage water using reverse osmosis (RO) helps solve water scarcity and waste disposal problems.
• Rainwater harvesting and sewage recycling in cities help conserve water and reduce sewage generation.